1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers (VCSELs). More specifically, it relates to VCSEL configurations that are particularly suitable for use at long wavelengths.
2. Discussion of the Related Art
VCSELs represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, the various material systems can be tailored to produce different laser wavelengths, such as 1550 nm, 1310 nm, 850 nm, 780 nm, 670 nm, and soon.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAS) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the GaAS substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 (another confinement layer) is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAS cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonate at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that provides current confinement. The protons can be implanted, for example, in accordance with the teachings of U.S. Pat. No. 5,115,442, which is incorporated by reference. The oxide layer can be formed, for example, in accordance with the teachings of U.S. Pat. No. 5,903,588, which is incorporated by reference. The insulating region 40 is usually formed either by implanting protons into the top mirror stack 24 or by providing an oxide layer. The insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path through the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that the current flows through the conductive central opening 42 to the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the VCSEL 10.
It should be understood that FIG. 1 illustrates a common VCSEL structure, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added. Because of the wide variety of VCSELs that are possible it is convenient to categorize VCSELs so that useful comparisons can be made. Typical categorizes include substrate material, output geometry (top-emitting or bottom-emitting), current isolation method, and electric contact configurations.
The substrate material that is used effectively controls the bottom DBR and the active region 20. This is because the bottom DBR must be well lattice-matched to the substrate since the active region, which must lattice match with the bottom DBR, cannot tolerate defects caused by a large lattice-mismatching. Commonly used substrate-DBR configurations include GaAs substrates with AlGaAs and/or AlGaInP; InP substrates with AlGaAsSb, with AlGaInAs, with InGaAsP, and/or AlGaPSb; and InAs/GaSb substrates with AlGaAsSb, and/or AlGaSbP.
Top-emitting VCSELs (in which light is emitted through a top DBR) have the advantage of being compatible with standardized 850 VCSEL packages, but the disadvantages of being less compatible with dielectric, oxide, metamorphic and metal-assisted DBRs. Bottom-emitting VCSELs (in which light is emitted through a bottom DBR) have the advantages of being compatible with a wide range of top DBR materials and being compatible with co-planar transmission lines, but the disadvantage of being less compatible with current 850 nm VCSEL packaging.
Methods of providing current isolation include ion-implantation and oxide aperture structures, including pillars, holes, and trenches. Ion-implanted VCSELs have demonstrated greater reliability than those that use oxide apertures. However, oxide-apertured VCSELs have advantages of higher speed and higher efficiency. Both schemes are suitable for long-wavelength VCSELs.
Various anode and cathode electrical contacting schemes are possible with VCSELs. Placing electrical contacts on opposite sides of the substrate reduces manufacturing difficulty. However, having all electrical contacts on the same side of the substrate can reduce device capacitance, and thus improve high-speed characteristics. Also, having both electrical contacts on the same side of the substrate enables the use of an insulative DBR on the opposite side of the substrate. Such an insulative DBR does not require doping, which enables DBR compositions with bandgaps close to the emission wavelength. This can boost reflectivity.
While generally successful, VCSELs have problems. In particular, VCSELs used at long wavelengths, such as 1550 nm or 1310 nm, are currently significantly less than optimal. This is a problem because long-wavelength VCSELs (1.2 μm-1.7 μm) are needed for future generation data communication and telecommunication applications. Therefore, novel VCSEL structures suitable for use in long-wavelength applications would be beneficial.